It is known that different fluids, such as coatings, absorb different, specific bands of IR (infrared) radiation. This knowledge is used in radiometric measuring where absorption of radiation wavelengths within the IR area is used. The main or other absorption bands of a fluid to be measured is used in that the change of energy when the radiation passes through the film is analysed. An important condition for such a measurement is that the substrate on which the fluid is received reflects a major part of the radiation in the selected wavelength range. Thus, by directing IR radiation towards a film, such as a coating layer on a substrate it is possible to determine the thickness of the coating layer.
If the substrate receiving the coating has low reflection it may be difficult and in some cases even impossible to measure directly on the coating of the substrate. Also for substrates having an uneven surface or having surface with other considerable variations there may be problems with measuring directly on the coating of the substrate.